Magnetic resonance imaging (henceforth abbreviated as MRI) apparatuses are diagnostic imaging apparatuses for medical use, which utilize nuclear magnetic resonance phenomenon of, mainly, protons. MRI apparatuses enable non-invasive imaging of an arbitrary section, and enable acquisition of morphological information and information on biological functions such as blood flow and metabolic functions. In general, on a test subject placed in a static magnetic field, a slice gradient magnetic field and a radio frequency magnetic field of a specific frequency are simultaneously applied to excite nuclear magnetization in a section desired to be imaged. A phase encoding gradient magnetic field and a read-out gradient magnetic field are applied on the excited nuclear magnetization to impart two-dimensional positional information, and magnetic resonance signals (echoes) generated by the nuclear magnetization are measured. The measurement of the magnetic resonance signals is repeated until the measurement space called k-space is filled. The signals filled in the k-space are subjected to inverse Fourier transform, and thereby converted into an image.
Pulses for generating echoes and gradient magnetic fields are applied according to a pulse sequence set beforehand. As this pulse sequence, various pulse sequences for various purposes are known.
One of the important diagnostic images obtainable by MRI is diffusion-weighted image (DWI). DWI is an image in which self-diffusion of water molecules contained in a biological tissue is emphasized. A lesion of cerebral infarction at an acute stage immediately after onset can be imaged as DWI, and it is known that it shows contrast different from those of T1-weighted image and T2-weighted image. DWI is obtained by applying MPG (motion probing gradient), which induces reduction of signal intensity by dephasing, on nuclear spins in random motions of a subject, and then measuring echoes to obtain signals corresponding to diffusion rate of the nuclear spin. Since the reduction of signal intensity due to dephasing is caused by nuclear spin diffusing in the direction of the application of MPG, diffusion information for an arbitrary direction can be obtained by controlling the application direction of MPG. Further, diffusion weighting degree can be adjusted by varying the diffusion factor (b value), which is a parameter relating to application intensity and time of MPG, and an image of a higher diffusion weighting degree can be obtained with a higher b value.
As a technique for measuring spatial diffusion distribution of water molecules, there is DTI (diffusion tensor imaging). DTI is based on assumption of a normally distributed three-dimensional elliptic diffusion model, and is widely used as a technique for analyzing degeneration of tissues or structure of nerve tract of white matter by calculating mean diffusivity (MD), and diffusion fractional anisotropy (FA). The pulse sequence for DTI is constituted so that pulse sequence of DWI is repeated with changing the application direction of MPG. Since this pulse sequence requires calculation of components of diffusion tensor, the measurement is performed by successively applying MPG for 6 or more non-parallel independent directions.
Diffusion kurtosis imaging (DKI), which is based on assumption of a non-normally distributed diffusion model, has also been proposed in recent years as a technique for emphasizing degree of diffusion movement restriction caused by cell membranes, intracellular organelles, and so forth. This technique is expected as a technique for capturing change of microstructures accompanying tissue degeneration or cell proliferation in contrast to DTI in which a normally distributed diffusion model is assumed. The pulse sequence of DKI is constituted so that the pulse sequence of DTI is repeated with changing the b value. Since components of the diffusion tensor and kurtosis tensor are calculated in this technique, it is necessary to perform the measurement by applying MPG in 15 or more non-parallel independent directions with three or more of different b values.
In the measurement of DTI and DKI, body motion of a patient under imaging generally causes a computational error in an image reconstructed after the measurement, for example, computational errors of MD (mean diffusivity) and FA (diffusion fractional anisotropy). Although an image not influenced by body motion is desired, it may be difficult to distinguish computational error caused by body motion from signal change caused by pathological change, and it is difficult to determine presence or absence of body motion during imaging only from a calculated image. Further, in DWI, contrast of image changes with change of the application direction of MPG, and therefore it is difficult to detect presence or absence of body motion and correct it only from simple comparison of DWIs obtained with different MPG application directions.
Concerning this problem, for example, Non-patent document 1 proposes a method for performing correction for body motion in DTI by adding pulses for measuring data for the correction for body motion to the pulse sequence of DWI. Non-patent document 2 proposes a method of detecting body motion of a patient under imaging with an optical camera attached to a receiver coil and correcting it by using a checkerboard attached to the patient, which serves as a target.
Patent document 1 discloses a technique for detecting changes of position and direction of a patient (body motion) by obtaining three diffusion-weighted images as one group with three MPG application directions perpendicular to one another, obtaining a mean diffusivity image (trace-weighted MR result image) from the three diffusion-weighted images by calculation, and comparing it with a previously obtained mean diffusivity image. In this technique, while diffusion-weighted images are obtained for a plurality of groups of different combinations of the three application directions of MPG, mean diffusivity images are calculated, and body motion is temporally detected. It also discloses correction of diffusion-weighted images according to detected body motion (position and direction).